Therapeutic exercise programs for osteoarthdtis (OA) of the knee play a major role in conservative management of OA. Strengthening of the quadriceps and hamstrings is one of the primary objectives of these exercise programs, yet the mechanisms of how strengthening affects the Osteoarthritic knee are not well understood. It is speculated that the positive influence of strengthening the knee musculature for OA and other knee joint problems is due to the strengthened muscle's ability to "stabilize" or "reduce stress" or somehow "protect" the knee joint. This commonly held belief has not been assessed experimentally. The purpose of this study is to elucidate the mechanisms of how strengthening primary knee joint musculature may cause biomechanical adaptations that directly influence knee joint mechanical loading. The specific aims will be to determine knee joint biomechanical forces and moments during level walking and stair climbing in patients with OA and age matched persons without OA, to strength train quadriceps and hamstrings musculature for both groups, and then to compare knee joint contact forces and moments before and after training in both groups. The PIs hypothesize that strength of the quadriceps and hamstrings has little influence on knee joint mechanics in patients that have strength sufficient for the functional tasks of walking and stair climbing. They believe that only patients with significantly weaker musculature compared to age-matched controls will show joint biomechanical changes due to strength training and that increased strength does not necessarily "protect" the joint. Two groups (OA patients and age-matched controls) will undergo a 2-month supervised strength-training program. Demographic, psychosocial, and biomechanical measures will be collected pre and post training. Three-dimensional gait analysis will be used during level walking and stair climbing. EMG will be used to determine muscle activation. Muscle forces and joint kinetics will be estimated using SIMM musculoskeletal modeling software and inverse dynamics. Repeated measures ANOVA and ANCOVA will be used to test the study hypotheses. The results of this study will support national objectives to promote evidence based arthritis interventions by providing answers pertaining to whether and how a therapeutic intervention directly affects the mechanical dysfunction associated with OA of the knee. [unreadable] [unreadable]